Attempts to cover stains and blemishes on architectural substrates to be painted have traditionally been performed using liquid paints or primers that are either solvent- or water-based. The use of liquid stain-blocking coatings, in general, has several disadvantages, some of which are disadvantages common to all liquid coating products, others of which are disadvantages specific to the use of these products to block the wide variety of stains that are known to occur.
Disadvantages that are common to most liquid coating products include the presence of volatile organic compounds (VOCs), the odor, the need for subsequent cleanup, and the drying time that is necessary prior to applying subsequent liquid coating layers. Disadvantages that are specific to the use of liquid coating products as stain blocking agents include those resulting from the interaction of the liquid coating with the stain while the coating is wet, as well as those arising from the interaction of the dried coating with the stain after the coating has dried, such as those that are known to occur when further liquid coating layers are applied.
A wide variety of stains are known to occur on substrates to be painted, making it difficult to select the proper liquid stain-blocking coating. Indeed, a liquid coating that is satisfactory to block all the various types of stains normally encountered has yet to be developed, due in part to the varying physicochemical properties of the various stains.
Typical stains for which stain blocking paints are used include inks, crayons, lipstick, grease pencils, smoke residue, tannins, and the like. These stains may be found on residential or commercial walls as graffiti, on wooden substrates, wood-composite substrates, concrete substrates, paper substrates (such as wall board coverings) and other such substrates that are normally painted with liquid coatings. The stains may reside either on the surface of the substrate itself, or near the surface of the substrate, or on the surface of a paint layer previously applied to the substrate, or well below the surface of the substrate. These stains may have components such as dyes, conjugated organic compounds, aromatic color bodies, and the like, and may be soluble in organic liquids, in water, or in both. Wood containing knots can be a source of stains also.
Water-based paints, including the so-called stain-blocking paints, readily redissolve water-soluble stains, and allow them to migrate to the surface so that they are still visible, often even after numerous applications. Water-soluble inks are particularly difficult to block with water-borne paint, because the ink dye is freely soluble in the water (continuous medium) of the paint. Even when dry, subsequently applied liquid coating layers again redissolve these water-soluble stains, leading to the stain being present and visible even after several water-based paint applications.
Similarly, solvent-based paints may redissolve lipophilic stains. The colored compounds or polymers of such stains may then be transported through the liquid film before the film has set to a dry state. Diffusion of these compounds through very thin films (often coatings are applied at about 25 to about 375 micrometers [about 1 to about 15 mils] thickness) typically occurs very rapidly, certainly within minutes, from the stain source to the top of the applied film. Stains or stain color bodies may thus travel through an applied liquid paint or primer due to its solubility in the continuous medium of the applied paint or primer (water and/or an organic solvent).
Wood containing knots can be especially troublesome. Coating such wood with a paint containing a solvent for the dark knot's color bodies can cause the colored chemicals to migrate to the newly formed paint surface. Thus, in a fashion similar to that of other lipophilic stains, knots in wood can cause the wooden substrate to be difficult to paint to hide the knot and its color bodies. It is then difficult to gain a uniform appearance, even with multiple coats of paint, since the color bodies from the knot will continue to be solubilized and transported to the freshly painted surface due to the solubility of the color bodies in the paint's continuous phase.
In order to limit the porosity associated with stain migration in a dried latex film, less porous latex primer films are commonly formulated by adding more coalescing solvent to the liquid paints, resulting in a better coalesced film, but one having a higher VOC content. Or if lower Tg or lower molecular weight polymers are used to obtain better films at ambient temperature with minimal amounts of solvent, the resulting films may be tacky and prone to dirt pickup. Thicker coatings which may slow stain transmission may have undesirable characteristics such as poor performance on vertical applications (poor sag resistance). Selecting a stain-blocking composition without regard to the effects that result from the drying of the composition would clearly be an advantage.
Typically, when a liquid stain blocking primer paint is applied to a substrate stained with a soluble colored material, the stain is solubilized and transmitted very rapidly through the drying paint film, often within seconds. Thus the primer paint is not blocking the stain, since it is transmitted rapidly through the wet coating. If the primer coating, when dry, does not hide the colored compound distributed throughout the coating, but nonetheless traps the stain within the coating beneath the coating surface, it can at best be considered a stain-trapping formulation, and not a stain-blocking formulation. In such cases, the color will still be visible on the dry primer surface. If the primer coating is an excellent stain trapper, it will inhibit the transport of the colored stain through subsequently applied liquid coatings, but often perhaps still leaving the stain visible. Nonetheless, some of the better stain trapping latex primer paints on the market today do not completely trap water-soluble stains.
It would clearly be an advantage to obtain a stain-blocking effect without the drawbacks associated with liquid paint or primer stain-blocking coatings. It would be especially advantageous were it possible to block stains regardless of the hydrophilic or lipophilic nature of the stain, and without the need to wait until the liquid paint or primer has dried before subsequently applying one or more additional liquid coating layers. Providing a relatively non-porous coating that is less susceptible to bleed-through when subsequent liquid coating layers are applied would be a further advantage.